1. Field of the Invention
This invention relates to a method for fabricating a semiconductor device and, more particularly, to an etching method used in the method for fabricating a semiconductor device.
2. Description of the Related Art
To constitute various types of semiconductor devices including, for example, laser diodes, GaAs-FET and HEMT, a hetero junction structure has been used to improve the characteristics of the semiconductor device. In order to arrange a good-quality hetero junction structure, the lattice constants of both materials should be substantially coincident with each other.
In the field of semiconductor lasers, for example, there has been proposed the reduction of a threshold current by use of a so-called heterojunction structure wherein an emission region having a small band gap energy be sandwiched between semiconductor layers having a large band gap energy. Since then, it has been known to prepare a heterojunction structure of good quality using GaAs and AlxGa1xe2x88x92xAs, thereby forming a GaAs-AlGaAs double heterojunction laser.
The AlxGa1xe2x88x92xAs material exhibits an increasing band gap energy Eg with an; increase in x, and, although the refractive index n decreases, the change in lattice constant is very small.
In the selective etching at the interface of growth of a hetero junction structure for the formation of a semiconductor device having the hetero junction structure, the etching is stopped only due to the difference in chemical etching rate between a layer to be etched and a layer for stopping the etching. Accordingly, it is necessary that the ratio in etching rate between the layer to be etched and the layer for stopping the etching be 50 or more.
This makes it difficult to stop the etching with high accuracy depending on the types of materials for the layer to be etched and the layer for stopping the etching in the course of selective etching, even though an AlxGa1xe2x88x92xAs material capable of forming a high-quality heterojunction is used. This entails much time for the choice of an etching solution and also an appreciable limitation placed on the choice of materials used to constitute a semiconductor device.
FIG. 13 is a perspective view showing a conventional semiconductor laser. In FIG. 13, indicated by 100 is a semiconductor laser, by 101 is an n-type GaAs substrate (n-type is hereinafter referred to as xe2x80x9cn-xe2x80x9d, and likewise, p-type is referred to as xe2x80x9cp-xe2x80x9d), by 102 is a buffer layer made of n-GaAs, by 103 is an n-type clad layer made of n-Al0.5Ga0.5As, by 104 is a multiple quantum well active layer made of Al0.35Ga0.65As/Al0.15Ga0.85As, by 105 is a first p-type clad layer made of p-Al0.5Ga0.5As, by 106 is an etching stopper layer made of p-Al0.2Ga0.8As, by 107 is a current block layer made of Al0.6Ga0.4As, by 108 is an opening of the current block layer 107, by 109 is a surface protective layer made of n-GaAs, by 110 is a second p-type clad layer made of p-Al0.5Ga0.5As, by 111 is a contact layer made of p-GaAs, by 112 is a removed region of the contact layer 111, by 113 is a p electrode, and by 114 is an n electrode.
Next, a method of fabricating a conventional semiconductor laser is described.
FIGS. 14 and 15 are, respectively, a sectional view showing a semiconductor laser at one stage in a conventional method of fabricating a semiconductor laser. FIGS. 14 and 15 are, respectively, a sectional view taken along line XIVxe2x80x94XIV of FIG. 13.
Referring to FIG. 14, after successive deposition, on the n-GaAs substrate via the buffer layer 102, of the n-type clad layer 103, the multiple quantum well active layer 104, the first p-type clad layer 105, the etching stopper layer 106, the current block layer 107 and the surface protective layer 109, a resist film is formed on the surface of the surface protective layer 109 to form a resist pattern having a band-shaped opening along a direction of an optical waveguide. The surface protective layer 109 is subjected to patterning by use of a photolithographic technique using the resist pattern as a mask. Subsequently, after removal of the mask pattern of the resist film, the current block layer 107 is selectively etched through the mask of the patterned surface protective layer 109 until the etching stopper layer 106 is exposed, thereby forming a band-shaped opening 108 in the current block layer 107.
Thereafter, the second p-type clad layer 110 and the contact layer 111 are successively built up on the current block layer 107 including the opening 108 and the surface protective layer 109.
A resist film is formed on the surface of the contact layer 111, and a resist pattern 115 having an opening is formed in the vicinity of opposite end faces of the band-shaped opening 108, followed by selective etching of the contact layer 111 through the resist pattern 115 used as a mask to form the removed region 112 of the contact layer 111. The results provided by the selective etching step are shown in FIG. 14.
For selective etching and removing the contact layer 111, a mixture of ammonia and hydrogen peroxide is used as an etching solution.
With reference to FIG. 15, the resist patter 115 is removed and the p electrode 113 is formed, and the n-GaAs substrate 101 is polished at a back side thereof to a given thickness, followed by formation of the n electrode 114. The results of these steps are shown in FIG. 15.
The etching solution (a mixed solution of ammonia and an aqueous hydrogen peroxide solution) used in the selective etching for removal of the contact layer 111 carried out in the conventional fabrication method serves to stop the etching by using only the difference in chemical etching rate between GaAs used for the contact layer 111 and Al0.5Ga0.5As used for the second p-type clad layer 110. Accordingly, it is necessary that the ratio of the etching rate between the GaAs of the layer to be etched and the AlxGa1xe2x88x92xAs of the etching stop layer be 50 or over. To this end, it is necessary that the compositional ratio of Al in the etching stop layer be at 0.2 or over.
In this case, the etching stop layer is constituted of the second p-type clad layer 110, so that the compositional ratio of Al can be set at 0.5, thereby ensuring a satisfactory etching rate ratio to GaAs. In general, however, such conditions are not always ensured, and thus, the selective etching of a compound semiconductor subjected to hetero junction has suffered a substantial limitation depending on the type of heterojunctioned material, which has, in turn, placed considerable limitations on the selection and structure of constituting materials of a semiconductor device.
Though a satisfactory etching rate ratio has been ensured, the control of carrying out etching to a necessary and sufficient extent is quite difficult, under which overetching leads to side etching. If a mixed solution of ammonia and an aqueous hydrogen peroxide solution is provided as an etching solution, surface oxidation takes place violently, thus being undesirable from the standpoint of surface morphology.
It will be noted that known techniques are described in Japanese Patent Laid-Open Nos. Hei 01-099276, Sho 61-077384 and Sho 62-176183, which disclose techniques of improving the accuracy of selective etching.
In Japanese Patent Laid-Open No. Hei 01-099276, there is disclosed a method using tartaric acid as an etching solution.
Moreover, in Appl. Phys. Lett. 55(10), Sep. 4, 1989, p. 984-p. 986, photochemical etching is described wherein a laser beam from a GaAs/AlGaAs hetero structure is irradiated.
The present invention has been made to overcome the above-described drawbacks and disadvantages of the related art. It is an object of the present invention to provide a method for fabricating a semiconductor device comprising the step of accurately stopping selective etching at the interface of a hetero junction arrangement.
According to one aspect of the invention, there is provided a method for fabricating a semiconductor device as follows. The method comprises the steps of; providing a hetero junction structure wherein a first semiconductor layer is formed thereon with a second semiconductor layer that has a band gap smaller than that of the first semiconductor layer and a valence band energy larger than that of the first semiconductor layer and forming a metal film on a surface of the second semiconductor layer and outside a first portion where the second semiconductor layer is to be removed; and forming a mask pattern covering the metal film and permitting the first portion of the second semiconductor layer to be exposed and selectively removing the second semiconductor layer under irradiation of light through the mask pattern as a mask by use of an etching solution having a Fermi level higher than that of the second semiconductor layer.
Accordingly, holes contributing to etching are generated by irradiation of light and the second semiconductor layer is rendered thinner. Thus mobility of the holes in directions parallel to the thin film of the second semiconductor layer increases and the holes are likely to move toward the metal film via the second semiconductor, thereby reducing the number of holes contributing to the etching and stopping the etching of the second semiconductor layer. The etching can be stopped to a necessary and sufficient extent at the interface of the hetero junction, thereby permitting selective etching. Thus, the stop of the etching can be very accurately controlled. Eventually, this leads to mitigation of limitations on the type of material and structure of a semiconductor device and also to inexpensive fabrication of semiconductor devices having uniform characteristics by a simple procedure.
Another object of the invention is to provide a method for fabricating the AlGaAs laser having the contact layer-removed structure by a simple procedure.
According to another aspect of the invention, there is provided a method for fabricating a semiconductor device. The method comprises the steps of: successively forming, on a GaAs substrate of a first conduction type, a lower clad layer made of an AlGaAs material of a first conduction type, an active layer made of an AlGaAs material and having a multiple quantum well structure, a first upper clad layer made of an AlGaAs material of a second conduction type, and a current block layer made of an AlGaAs material of a first conduction type and forming a groove for a current path in the current block layer made of the AlGaAs material along a direction of a light guide; forming a second upper clad layer made of an AlxGa1xe2x88x92xAs (0 less than xxe2x89xa61) on the current block layer so as to bury the groove, and forming thereon a contact layer made of an AlyGa1xe2x88x92yAs (0xe2x89xa6yxe2x89xa61 and y less than x) of a second conduction type; forming a metal electrode film on the contact layer and forming a resist pattern so as to cover the metal electrode film therewith and expose a surface of the contact layer at opposite ends of the groove for the current path; and selectively etching the contact layer with an etching solution, which has a Fermi level higher than that of the contact layer and contains tartaric acid, by use of the resist pattern as a mask under irradiation of light.
Accordingly, the selective etching of the contact layer of the AlGaAs laser having a contact layer-removed structure can be performed by a simple process. This eventually leads to the mitigation of limiting conditions of the type of a material and structure constituting the AlGaAs laser having the contact layer-removed structure and also to the inexpensive fabrication of AlGaAs lasers, which have uniform characteristics, good surface morphology and the contact layer-removed structure, by a simple procedure.
Other objects and advantages of the invention will become apparent from the detailed description given hereinafter. It should be understood, however, that the detailed description and specific embodiments are given by way of illustration only since various changes and modifications within the scope of the invention will become apparent to those skilled in the art from this detailed description.